Late Cretaceous-Cenozoic exhumation of the Yanji area, northeast China: Constraints from fission-track thermochronology

The Yanji area, located at the border of China, Russia, and Korea, where the Phanerozoic granitoids have been widely exposed, was considered part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast. In this study, the cooling and inferred uplift and denudation history since the late Mesozoic are intensively studied by carrying out apatite and zircon fission-track analyses, together with electron microprobe analyses (EMPA) of chemical compositions of apatite from the granitoid samples in the Yanji area. The results show that: (i) zircon and apatite fission-track ages range 91.7-99.6 Ma and 76.5-85.4 Ma, respectively; (ii) all apatite fission-track length distributions are unimodal and yield mean lengths of 12-13.2 mu m, and the apatites are attributed to chlorine-bearing fluorapatite as revealed by EMPA results; and (iii) the thermal history modeling results based on apatite fission-track grain ages and length distributions indicate that the time-temperature paths display similar patterns and the cooling has been accelerated for each sample since ca 15 Ma. Thus, we conclude that sequential cooling, involving two rapid (95-80 Ma and ca 15-0 Ma) and one slow (80-15 Ma) cooling, has taken place through the exhumation of the Yanji area since the late Cretaceous. The maximum exhumation is more than 5 km under a steady-state geothermal gradient of 35 degrees C/km. Combined with the tectonic setting, this exhumation is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the late Cretaceous.